Rename util.c to string.c and introduce string.h.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/clock.c b/src/clock.c
index cc962e4..fc91db7 100644
--- a/src/clock.c
+++ b/src/clock.c
@@ -14,6 +14,7 @@
 #include "bregs.h" // struct bregs
 #include "biosvar.h" // GET_GLOBAL
 #include "hw/usb-hid.h" // usb_check_event
+#include "string.h" // memset
 
 // RTC register flags
 #define RTC_A_UIP 0x80